Since the beginning of 1970, NMR (Nuclear Magnetic Resonance) is widely utilized as a medical diagnostic apparatus, especially as an imaging means capable of providing soft organization imagings having high resolution and contrast without using detrimental x-ray.
That is to say, many atoms have a certain property called as spin to which small magnetic moment is attached.
When the outer magnetic field does not exist, configuration of a magnetic moment is irregular, but in the presence of static magnetic field, nuclear magnetic moment takes precession to approximately the magnetic field direction, so that net alignment is generated in the magnetic field. NMR imaging method is achieved by using this priciple. According to NMR imgaging method, when a short radio frequency pulse is oscillated from a coil surrounding a patient which is set in a static magnetic field, a configuration based on the new magnetic field and precession in phase are generated by this pulse. On the other hand, when oscillation of the pulse is stopped, the above moment returns to the distribution of alignment and the irregular distribution of precession phase on the basis of the former static magnetic field. In such a case, detectable nuclear magnetic resonance is generated at the receiving coil, and by measuring such NMR signals, a proton density map of the objective tissue can be represented. Also, the NMR signal is largely depended with parameters of spin-lattice relaxation time (T.sub.1, i.e. the time specific to return of nuclear magnetic moment to balance alignment in static magnetic field) and spin-spin relaxation time (T.sub.2, i.e. the time specific to return the nuclear magnetic moment to the irregular precession phase distribution). Therefore, these mesurements can be applied to the diagnosis of pathogenic tissue states of a patient.
In NMR imaging method, it is known that physical parameters such as temperature, viscosity and hydration or the like of the tissue is effective to increase NMR signal strength or to change the contrast an NMR image. However, these methods are apparently not suitable for clinical applications. A method for enhancing the contrast of NMR images which is known in the present stage using a paramagnetic compound, as a contrast agent, which decreases spin-lattice relaxation time (T.sub.1) at low concentration thereof, and decreases spin-spin relaxation time (T.sub.2) at high concentration thereof. Contrast agents have been researched, and a typical example of such contrast agents are inorganic paramagnetic salts such as iron, manganese, chromium; or a organic chelate complex which consists of the paramagnetic metal ion mentioned above and one of various complex forming agents which are usually are aminopolycarbxylic acid such as ethylenediaminetetraacetic acid or diethy lenetriaminepentaacetic acid. The contrast agent is taken orally or otherwise in the form of a solution or a colloidal dispention liquid.
However, all of the known contrast agents which are suggested are found to be insufficient practically for use in NMR imaging methods, e.g., due to the difficulty in preparing such agents in a pharmaceutically acceptable form, a lack stability of the pharmaceutical form, difficulties in oral administration, poor taste, toxicity, or the like and, and ineffective viewing for using as a contrast agent, e.g. due to accuracy, clearness.